Duct Sizing Question

[Richard]

If I'm using 8" round duct to facilitate my returns, shouldn't the register boots and the takeoffs have the same area as the 8" pipe?

I have the same question with the supply boots as well. If the area of 8" round is

100 square inches, why then are there so many boots with dimensions like:

4 x 10, 6 x 10, 8 x 10 ?

It appears none of these boot openings or takeoffs have adequate cross-sectional areas to facilitate the round duct size I'm running to them. Won't the air volume be limited by the smallest piece in the branch?

I do realize this question is probably stupid, but my brain short circuited on this for some reason.

Thanks in advance for any help.

Richard

[daytona°]

were is you air duct calculator?

8" rd duct only has a 220 cfm capability with .10 static so, (2) 6" returns will be the max or you will get whistling

[Steve Scott]

[gofish]

please explain how you go about doing that when a variable speed furnace blower will vary its delivered cfm from 50 to 100%. :)

[James]

The duct has an air flow resistance related to it's size and length. It may be legitimate to have a bigger cross section on a long run to improve the air flow even though there is a lower cross section at the end.

[Steve Scott]

You're a PITA, Fish. :) But to answer your question you design for the max air flow, but then you knew that.

IMO, the trick really comes > Steve Scott wrote:

[udarrell]

The linked pages below may not be perfect, but might help answer some individuals' questions.

- udarrell